1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method, a program and a computer readable information recording medium, and in particular, to an-image forming apparatus, in which positional shift occurring upon image forming operation can be effectively inhibited, an image forming method executed in the apparatus, a program including instructions for executing the method by control of a computer and a computer readable information recording medium storing the program.
2. Description of the Related Art
Many manners are known for an image forming process applied in an image forming apparatus for forming a color image with electrophotographic technology. As one thereof, a tandem type one is known.
In this type, photosensitive bodies for respective color images to produce are provided, together with image forming process elements corresponding to the photosensitive bodies. In this configuration, these photosensitive bodies and image forming process elements are disposed along an intermediate transfer member or a paper conveyance belt, images produced therewith for the respective colors are overlaid together on the intermediate transfer member, or the color images produced on the postpositive bodies are transferred to paper each time the paper conveyed by the paper conveyance belt passes through a respective one of transfer processes of the respective photosensitive bodies, and thus, a full color image is produced as a result of all the transfer stations being passed therethrough.
FIG. 3 shows one example of a configuration of an image forming unit PTR of a color image forming apparatus in the latter one of the tandem type image forming apparatus, disclosed by Japanese Laid-open Patent Application No. 2004-237623.
In the figure, photosensitive drums 6a through 6d for forming images of different colors (yellow, magenta, cyan and black, which may be simply referred to as alphabets, i.e., Y/y, M/m, C/c and K/bk, respectively, hereinafter) are disposed in one row along a conveyance belt 10 conveying transfer paper.
According to image signals for recording, an exposure unit 5 emits laser beams modulated by the respective Y, M, C and K recording image signals, which scan the photosensitive drums 6a through 6d, respectively, which are previously uniformly electrically charged by a charger. Thus, electrostatic latent images are produced on the photosensitive drums 6a through 6d, respectively. The respective electrostatic latent images are then developed with Y, M, C and K toners in respective developers 7a through 7d, and thus, the toner images in the respective colors are obtained.
On the other hand, transfer paper conveyed to an intermediate transfer belt 10 of a transfer belt unit, from a paper feeding cassette 8. Then, on the transfer paper 8 thus placed on the intermediate transfer belt 10, the respective color toner images are transferred in sequenced by transfer units 11a through 11d, respectively, from the respective photosensitive drums 6a through 6d, while the transfer paper moves to pass by the respective photosensitive drums 6a through 6d in sequence as a result of the transfer paper being carried by the intermediate transfer belt 10.
Thus, on the transfer paper, the respective color toner images are overlaid together, and thus, a full color image is produced on the transfer paper.
After that, a fixing unit 12 is applied to fix the full color image, thus produced on the transfer paper, to the transfer paper. After that, the transfer paper is ejected from this machine PTR.
The intermediate transfer belt 10 is a translucent endless belt supported by a driving roller 9, a tension roller 13a and a following roller 13b. As the tension roller 13a is pressed to the belt 10 by means of a spring not shown, tension of the belt 10 is kept constant.
FIG. 4 shows a plan view of an optical unit of the exposure unit 5, shown in FIG. 3, viewed from the top.
In FIG. 4, light beams from a laser diode unit (simply refereed to as an LD unit hereinafter) 31bk and a LD unit 31y, including laser diodes and laser drivers which modulate the laser light of the laser diodes, pass through cylinder lenses 32bk, 32y, and are applied to a lower part of a polygon mirror 34 by means of reflective mirrors 33bk and 33y. Along with rotation of the polygon mirror 34, the polygon mirror 34 deflects the thus-applied light beams, which then pass through fθ lenses 35bkc and 35ym, and then are bent by first mirrors 36bk and 36y. 
Similarly, light beams from LD units 31c and 31m pass through cylinder lenses 32c, 32m, and are applied to an upper part of the polygon mirror 34. Along with rotation of the polygon mirror 34, the polygon mirror 34 deflects the thus-applied light beams, which then pass through the fθ lenses 35bkc and 35ym, and then are bent by first mirrors 36c and 36m. 
On an upstream side of a writing start position in a main scan direction, cylinder mirrors 37bkc and 37ym, as well as, sensors 38bkc and 38ym are provided. The light beams passing through the fθ lenses 35bkc and 35ym are then reflected and condensed by the cylinder mirrors 37bkc and 37ym, and thus are applied to the sensors 38bkc and 38ym. These sensors 38bkc and 38ym are synchronization detecting sensors for carrying out synchronization in the main scan direction.
On a downstream side of an imaging range, the same as the above-mentioned upstream side, cylinder mirrors 39bkc and 39ym, as well as, sensors 40bkc and 40ym are provided. The light beams passing through the fθ lenses 35bkc and 35ym are then reflected and condensed by the cylinder mirrors 39bkc and 39ym, and thus are applied to the sensors 40bkc and 40ym (also synchronization detecting sensors).
For the purpose of detecting the light beams from the LD units 31bkc and 31c, the sensors 38bkc on the writing start side and the sensor 40bkc on the writing end side, are applied commonly. Similarly, for the purpose of detecting the light beams from the LD units 31y and 31m, the sensors 38ym on the writing start side and the sensor 40ym on the writing end side, are applied commonly.
Since these two different color image forming light beams are applied to the same sensor, incident angles to the polygon mirror 34 are made different between the respective color light beams, and thus, timing at which the respective light beams are actually applied to each sensor can be changed. Thereby, the corresponding respective detected signals are output from each sensor in a form of a time-series pulse sequence. As shown in FIG. 4, scanning directions for K (bk) and C (c) are opposite to that for Y (y) and M (m).